Methods for the electrochemical testing of a specific parameter of conversion coatings on metal surfaces are known. For example, the article "Quantitative Test for Zinc Phosphate Coating Quality" by R. W. Zurilla et al., SAE Technical Paper No. 780178, Society of Automotive Engineers, Inc., Warrendale, Pa. (paper presented during the period Feb. 27-Mar. 3, 1978 at SAE Congress and Exposition, Cobo Hall, Detroit) discloses an electrochemical test for zinc phosphate coating porosity which the authors found correlated well with salt spray performance. In a paper entitled "Electrochemical Methods to Determine the Corrosion Rate of Coated Metals", by M. Piens et al. published in the Proceedings of the International Congress of Metal Corrosion, 8th, 1981, page 1021, Volume 2, 47LJAF, the authors discuss the advantages of impedance measurements over polarization resistance in gaining insight into the behavior of the coating and the corrosion mechanism.
However, no effective electrochemical system for the determination of multiple parameters of conversion coatings has heretofore been developed, nor has a method for using these results to control the composition of the coating bath, despite the obvious need for such a system in the metal coating industry.